Asparagus (Asparagus officinalis L.) is a well-managed/assisted crop, with good water and nutrient availability (Yeasmin et al., 2013). Asparagus decline is associated with both abiotic and biotic factors. Abiotic factors associated with the decline of asparagus production are related with the release of allelopathic compounds, including nutrient imbalance, and deterioration of physical and chemical conditions. Improving biotic conditions to promote asparagus growth was recently attempted by inoculation of beneficial microbes, such as AMF that had a high affinity for asparagus, and the symbiotic relationship promotes vigor in asparagus plants (Nahiyan and Matsubara 2012). Traditionally, asparagus has been grown in temperate climates, but in the past few years, commercial production of asparagus has begun in tropical climates. Thus, asparagus appears to have a wide environmental adaptability. To date, the growth and physiological responses of asparagus of high temperature has not been well researched.
Heat stress often decreases the uptake of nutrients in plant tissues or decreases the total content of nutrients in the plants, although effects can vary among nutrients and species (Giri et al., 2017). Hungria and Kaschuk (2014) reported that heat stress can also disrupt enzymes involved in nutrient metabolism (e.g., nitrate and ammonium assimilation). When a plant undergoes heat stress, several toxic reactive oxygen species (ROS) are generated in cells, and the oxidative stress caused by these ROS is one of the major damaging factors in plants (Wahid et al., 2007). To limit oxidative damage under stress conditions, plants have developed a series of detoxification systems involving several antioxidative enzymes, such as superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase, catalase, or glutathione reductase (Zhu et al., 2010). The mechanisms underlying the influence of AMF on the ROS metabolism of host plants under heat stress conditions remain uncertain.
AMF are well known to increase plant growth and yield, improve water and nutrient uptake, and to lessen abiotic stress factors (Mouk and Ishii, 2006). AMF establish mutualistic symbioses with most higher plants and benefit plants by increasing resistance to environmental stresses, enhancing plant nutrient acquisition, and improving soil quality (Hart et al., 2018). As a result of the increased nutrient uptake, plants inoculated with mycorrhizae frequently produce higher yields than do those without AMF. Moreover, AMF inoculations into the plantations of soybean, core, millet, trifoliate orange, rice, and 17 tropical legumes have been demonstrated with improved economic impact on agriculture and horticulture by enhancing plant growth and yield (Gholamhoseini et al., 2013). For these reasons, mycorrhizae can play an important role in agricultural production by creating the possibility of enhancing absorption of nutrients. Mycorrhizae particularly enhance P uptake but can also increase NH4+ and NO3− uptake, as well as the uptake of other nutrients, including Zn, Cu, and K (Hart et al., 2018). However, it has been unclear how antioxidative factors change through symbiosis with AMF in asparagus plants and how the changes are associated with heat tolerance. We hypothesized that 1) AMF will increase growth and nutrient uptake of the affected heat-stressed asparagus plants, 2) the increased activity of enzymatic antioxidants in mycorrhizal asparagus imply that the AMF symbiosis can alleviate ROS damage, protect plants against oxidation, and improve heat stress tolerance during plant production. Testing these hypotheses will extend our present understanding of plant responses to heat stress by examining the importance of AMF under the recent advent of global warming.
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